Viscosity Index Improvers (VII) are polymers that influence rheological properties of the lubricating oil by, for example, interactions with components of lubricating oils (e.g., base oil, pour point depressant (PPD), dispersant or detergent inhibitor (DI) package) and/or interactions with itself (polymer-polymer). VII interactions with lubricating oil components (e.g., wax) are well characterized by tests like Mini-Rotary Viscometer (MRV) (ASTM D4684), Pour Point (ASTM D5949, D5950, or D5985), and Scanning Brookfield (ASTM D5133).
Viscosity index improvers are typically polymeric molecules that are added to compositions such as a lubricating oil to alter viscosity response of the composition with temperature. With lubricating oils, these molecules thicken the oil at higher temperatures offsetting viscosity losses that would otherwise reduce the lubricating properties and wear protection of the oil and increase boundary friction. At low temperature, however, these polymers may form networks due to polymer entanglements or small crystals. The extent to which such networks are formed depends on the temperature, concentration, and the nature and molecular weight of the polymer. Such networks can “stiffen” the lubricating oil and reduce its flowability, form ‘lumps’ or ‘gels,’ or turn the entire solution into a ‘frozen’ solid (composition that can not be poured).
Certain VII having a high amount of continuous ethylene segments dispersed in the polymer backbone can lead to gelation via polymer network formation. Olefin copolymers such as ethylene-propylene copolymers, for example, tend to stiffen due to crystallization of the ethylene segments of the polymer chain. Other, non-crystalline molecules can form gels or become frozen solid when the polymer forms stable entanglements. Certain VII containing styrene blocks also lead to network formation by polymer interactions. Both effects depend on the molecular weight of the polymer and its concentration and temperature of the solution.
Polymer interactions are different than wax interactions in base oils and require different measurement technique(s). Methods mentioned above such as Mini-Rotary Viscometer (MRV), Pour Point, and Scanning Brookfield are not sufficiently sensitive or indicative of how polymers such as alpha-olefin copolymers would interact in lubricating oils. There is currently a need for a test that would reliably detect the predisposition of a given polymer to form gels or turn into solids when formulated into a composition.